Reflex hammers, also known as tendon hammers, are used by physicians to check the reflex, or muscle response, of patients. The traditional reflex hammer consists of a shaft with a handle at one end and a hammer head at the other end of the shaft. The head has a striking surface, typically made of hard rubber. A physician checks a patient's muscle response by striking the hard rubber end of the reflex hammer against the patient's body, typically at the knee, elbow, or forehead. The physician then visually checks for the reflex muscle response to the strike.
More advanced reflex hammers have an electronic triggering mechanism in the head of the hammer. The triggering mechanism provides an electrical signal at the time of impact. This electrical signal can be provided to a device for measuring the reflex response to the strike. In this way a quantifiable, versus visual, response may be obtained. For example, an electric reflex hammer may be used when a patient undergoes a diagnostic procedure involving an evoked potential (EP) recording. The electric reflex hammer provides a signal to the EP recorder at the moment of impact of the hammer against the patient. The EP recorder can then correlate the time of impact with changes that may occur in the physiological signals that the EP recorder is recording from the patient.
Electric reflex hammers typically contain a momentary contact switching mechanism that opens or closes when the hammer strikes the patient. This switching mechanism typically comprises two or more contacts activated by a spring mechanism. A wire extends from one of the contacts for connection of the reflex hammer to the electric measuring device. A second wire, also for connection to the measuring device, is attached to a second contact at the end of a spring. On impact of the hammer head with a patient, the spring compresses and the two contacts mate, completing the circuit between the wires and providing a signal to the measuring device.
This type of triggering mechanism, however, often results in inconsistent contact closure time and, therefore, inconsistent triggering signals among otherwise identical reflex hammers. This inconsistency results from how the contact is attached to the spring and how the wire is attached to the contact on the spring. How the wires and contacts are attached affects the operation of the closure of the contacts in response to a hammer strike. Soldering of the wire and contact to the spring adds weight to the spring, which affects its motion in response to a strike. Since the solder weight among reflex hammers will vary, the contact closure response will also vary. Attaching the contact and wire to the spring by a screw or other connection mechanism also adds weight, affecting the response of the spring. Additionally, the spring and the wires attached to the spring have a degree of tension. When the spring tries to compress in response to a strike this tension restricts the compression, thereby affecting the closure of the contacts. The combined effects of additional and inconsistent weight resulting from the way the contact and wire are connected to the spring, and the tension in the wire and spring itself, makes the contact closure inconsistent. This inconsistency affects the measurement of the response time of muscles to a strike of the hammer.